ASE B4 Structural Analysis & Damage Repair cheat sheet

Unitized (unibody) vehicles use welded frame rails, aprons, strut towers, rockers, and floor pan as stressed members; body-over-frame vehicles use a separate ladder frame that carries the load.

Steel categories are mild/low-carbon, HSS (HSLA, dual phase), AHSS (DP600 and above), and UHSS/martensitic boron (often 1,000+ MPa); repair rules follow the grade, not the panel's appearance.

Structural parts commonly include frame rails, front and rear aprons, strut/shock towers, rocker panels, floor reinforcements, hinge and latch pillars, and crossmembers.

Panel material is identified with a magnet (steel attracts, aluminum and plastic do not), OEM stampings, the VIN/certification label, and the OEM body repair manual before tools are chosen.

Factory weld count matters: compare visible spot welds and seam sealer to the OEM diagram; missing or non-factory welds indicate prior repair.

Cracks in undercoating, paint, or seam sealer often reveal underlying deformation that is invisible on the outer panel.

Indirect damage signs include uneven panel gaps, mismatched ride height, wrinkled aprons, and displaced hood or trunk alignment.

Before inspection, note SRS components, high-voltage systems, fuel lines, and sensors; these restrict where heat, cutting, and welding may be used.

Datum is the OEM horizontal reference plane; centerline is the vertical front-to-rear plane; height and symmetry readings are compared to these references.

Control points are OEM-specified holes or locations with published length, width, and height dimensions; measure them, not cosmetic panels, to judge structural movement.

Typical body tolerance is about ±3 mm (1/8 in); dimensions outside the OEM range require repair or replacement.

Movement types include mash (shortened), diamond (parallelogram), sag (lowered center), sway (shifted sideways), and twist (unequal heights).

Equal left and right diagonal measurements indicate a square structure; unequal diagonals indicate diamond or sway.

Tram and centerline gauges check point-to-point and symmetry distances; 3D computerized systems compare length, width, and height to OEM data.

Measure before pulling, during pulls, and after stress relief; a single measurement after repair does not show whether the structure moved progressively.

A vehicle within dimensional tolerance but with poor gap alignment may still have panel or mounting-point damage; combine measurement with visual inspection.

Anchor the vehicle at OEM control points and pull opposite the impact direction; multiple controlled pulls are preferred over one large pull.

Overpull is limited to the OEM allowance; release slowly to allow springback and re-measure after each pull.

Many OEMs prohibit heat on AHSS, UHSS, and martensitic steel; mild steel may allow limited heat, but never above the OEM temperature limit.

Sectioning is prohibited at OEM-stated locations, in crush zones, on UHSS, near suspension or steering mounts, and near SRS sensor mounts.

A sectioned hollow member requires the OEM-specified backing sleeve or insert; sleeve length, material, and weld or rivet pattern must match the procedure.

After straightening, relieve stress by relaxing the pull, tapping, and inspecting for new cracks; then reseal and apply corrosion protection.

Body-over-frame rails can be straightened on a frame machine or replaced; cracks in cast or forged suspension and steering parts require replacement.

Aluminum structural parts are generally replaced rather than heat-straightened; alloys vary widely, so the OEM procedure governs.

Remove factory spot welds with a spot-weld cutter, drill, or belt sander; cut only the weld and preserve the base metal of the remaining panel.

Test fit, clamp, and measure before welding; door and lid gaps typically run about 4-5 mm or the OEM-specified value.

Apply weld-through primer to mating surfaces before welding; it maintains electrical conductivity and protects the joint from corrosion.

OEM-specified joining includes STRSW, GMAW plug or stitch welds, structural rivets, self-piercing rivets, and structural adhesive, alone or combined.

Sectioning a new or used part follows the same OEM location, sleeve, and weld rules as any other sectioning.

Restore corrosion protection with zinc-rich primer on bare metal, seam sealer on joints, cavity wax inside hollow sections, and undercoating underneath.

Disconnect the battery and follow SRS procedures before welding or cutting near airbag sensors; do not route welding current through sensors or modules.

Salvage structural assemblies are acceptable only when the OEM permits it and the part is verified undamaged and correctly prepared.

GMAW on steel commonly uses 75% argon/25% CO2 (or 100% CO2) with ER70S-6 wire; wire and gas must match the OEM specification for HSS.

STRSW is the factory-preferred method for many structural joints; current, time, and electrode force are set per the equipment or OEM chart and tested on scrap.

Plug weld holes are commonly about 5/16 in (8 mm); the weld must fill the hole and fuse the lower panel without burning through.

Structural welds require full penetration, no porosity, no undercut, and no cold lap; a destructive test coupon verifies settings before welding the vehicle.

The heat-affected zone should be kept small by using skip or stitch welds and OEM weld spacing rather than continuous seams where not specified.

Aluminum requires 100% argon, a push technique, and dedicated wire, liner, and gun to avoid steel contamination; filler is typically 4043 or 5356.

Self-piercing and squeeze rivets need access to both sides and OEM tooling; rivet-bonding combines adhesive for load with rivets for alignment.

Remove galvanizing or coating from the weld zone, weld with adequate ventilation, and recoat with weld-through primer or zinc-rich paint.

Welding or heating near SRS sensors, modules, or high-voltage cables is prohibited; disconnect power and follow OEM isolation procedures.